Availability

From the first bonfire walk downwards, until the first worm attacks you from the wall. At this point walk off the cliff on the right side. You will land on a ledge with a door, which you will be able to open as soon as you have the Forgotten Key. Look for a corpse underneath the ledge and drop down towards it. From there you will be able to drop to an even lower ledge with a cave entrance. Inside this cave entrance you will find two Elite Giants. The second one will always drop the key and a Soul of a Giant.

Notes

Using a bow and large amounts of arrows one can hide inside on of the two smaller caverns in the area (preferably the one with the chest due to its larger size and lack of holes to fall into) and shoot arrows from within the relative safety of the smaller caverns although on occasion some of the Elite Giant's attacks will connect through the wall.

Fast way to the Forgotten Key: Rush the Dragonrider and get to Lonesome Gavlan in No-man's Wharf and buy 20 poison throwing knifes, buy the Silvercat Ring in Majula and go into the Gutter. From there, make a run for the Black Gulch and kill the giants with poison. If you hit all knifes, you will need to finish off one giant with melee, if you do not have another ranged option. 4 Knifes to poison a giant, applying poison 2 times almost kills them.